Excitation characteristics

X-Ray Emission and Fluorescence. X-ray analysis by direct emission foUowing electron excitation is of Hmited usefulness because of inconveniences in making the sample the anode of an x-ray tube. An important exception is the x-ray microphobe (275), in which an electron beam focused to - 1 fim diameter excites characteristic x-rays from a small sample area. Surface corrosion, grain boundaries, and inclusions in alloys can be studied with detectabiHty Hmits of -- 10 g (see Surface and interface analysis). 

Figure 15.19 Knee point of the excitation characteristic of a current transformer...

It is often advantageous to place the window in the side of a proportional counter so that the x-ray beam passes perpendicularly to the central wire even though this necessarily shortens the path length.21 A second window may be placed opposite the entrance window to permit escape of the unabsorbed x-rays, which might otherwise excite characteristic lines upon being absorbed by the counter walls. 

Electrons excite characteristic transitions in the sample, which can be studied by analyzing the energy loss suffered by the primary electrons. 

The line at approximately 600 nm has a long decay time of 1 ms. It is the strongest one in the titanite luminescence spectrum under 266, 355 and 532 nm (Fig. 4.33b,c), but its relative intensity is much lower under 514 nm excitation (Gaft et al. 2003b). It appears that from all lines found in titanite luminescence spectra only two weaker ones at 563 and 646 nm have similar kinetic and excitation characteristics with the line at 600 nm. Such a combination of luminescence lines is very typical for Sm ". Thus the emission spectrum... 

Further, in atomic spectrometry we must face the serious problem that the behaviour (atomisation/excitation characteristics) of the analyte in the calibration samples should be the same as in the future unknown samples where the analyte of interest has to be quantified, otherwise peak displacement and changes of the peak shape may cause serious bias in the predictions. Fortunately, many atomic techniques analyse aqueous extracts or acid solutions of the (pretreated) samples and current working procedures match the amount of acids in the calibration and treated samples, so the matrices become rather similar. Current practices in method development involve studying potential interferents. The analyte is fixed at some average concentration (sometimes studies are made at different concentrations) and the effects of a wide number of potential interferents are tested. They include major cations, anions and... 

Since the terminal groups do not affect the electronic excitation characteristics of the polyenes, these dyes are invariably very long, b-carotene involves 11 conjugate bond groups to achieve absorption at a wavelength of450 nm and 478 nm, e=135,000 and 120,000 respectively. Retinol, being much shorter, exhibits a peak absorption at 325 nm, e=52,480 in ethyl alcohol. 

Radiation from radioisotope sources can be used to excite characteristic X-rays in samples upon which the beam of radiation is directed. Detection and analysis of these X-rays yield information about the composition of the sample. This opens the field of analytical applications of X-ray fluorescence analysis. The most frequent applications are in the ore processing and the metal coating industries. 

The physics of the Z-ray-emission process presents an even more fundamental barrier to the usefulness of EDS in low-Z element analysis. The electron beam incident on a sample excites characteristic Z-rays (as distinct from bremstrahlung) via the radiative decay of core holes created as a primary event within the inner electronic structure of the element in question. However, these core holes can decay via competing channels, the most important of these being the Auger process in this, the energy associated with the neutralization of the core holes is transmitted to another electron in a shallower level, which is then ejected from the atom. The probability of radiative as opposed to Auger decay decreases with Z, so that for sodium the relative probabilities are 1 40, for carbon 1 400. Instrumental factors notwithstanding, this... 

The EDS type of X-ray spectrometer is commonly included as a part of SEMs and TEMs. The reason for using EDS rather than WDS is simply its compactness. With EDS in an electron microscope, we can obtain elemental analysis while examining the microstructure of materials. The main difference between EDS in an electron microscope and in a stand-alone XRF is the source to excite characteristic X-rays from a specimen. Instead of using the primary X-ray beam, a high energy electron beam (the same beam for image formation) is used by the X-ray spectrometer in the microscopes. EDS in an electron microscope is suitable for analyzing the chemical elements in microscopic volume in the specimen because the electron probe can be focused on a very small area. Thus, the technique is often referred to as microanalysis. 

The fluorescence factor (F) arises from the excitation of characteristic X-rays of the element to be analyzed by the characteristic X-rays emitted from matrix atoms. This phenomenon occurs when the characteristic X-rays from matrix elements have energies greater than that required for exciting characteristic X-rays of the element to be analyzed. Thus, the resulting X-ray intensities of the element to be analyzed will be enhanced by the fluorescence effect. The fluorescence factor is usually least important in the matrix factor, because fluorescence X-rays may not exist, or the concentration of elements emitting fluorescence X-rays may be small. 

The second method (ASTM D-4294, IP 477) uses energy-dispersive X-ray fluorescence spectroscopy, has slightly better repeatability and reproducibility than the high-temperature method, and is adaptable to field applications but can be affected by some commonly present interferences such as halides. In this method, the sample is placed in a beam emitted from an X-ray source. The resultant excited characteristic X radiation is measured, and the accumulated count is compared with counts from previously prepared calibration standard to obtain the sulfur concentration. Two groups of calibration standards are required to span the concentration range, one standard ranges from 0.015% to 0.1% w/w sulfur and the other from 0.1% to 5.0% w/w sulfur. 

Figure 12.8 Loss of excitation characteristic of an admittance relay. The diagram is drawn in the impedance domain.

De Luca, A. Natuzzi, F. Lentini, G. Franchini, C. Tortorella, V. Conte Camerino, D. Stereoselective effects of mexiletine enantiomers on sodium currents and excitability characteristics of adult skeletal muscle fibers. Naun5m. Schmiedebergs Arch. Pharmacol. 1995, 352, 653-661. 

These nanostructured materials have many exciting characteristics such as large pore sizes, high porosity, high surface areas, and wide range of pore sizes and topologies. 

The sintered block should have a light yellow color. Its excitation characteristics are the same as those of the corresponding sulfide phosphor, but the emission color is a yellowish-green. 

Figure 8.48 Illustration of the analyzed layer from the sample The incoming primary radiation excites the lower and middle parts of the sample as shown but does not excite the upper part since the sample is infinitely thick for the primary radiation. The excited characteristic radiation from the element in the middle part is reabsorbed within the sample. Only the signal from the layer near the surface is able to be detected by the detector.

---